About two hours after the winners crossed the line, there was a loud explosion on the north side of Boylston Street, just before the photo bridge that marks the finish line. Another explosion could be heard a few seconds later.

The Boston Marathon said that bombs caused the two explosions and that organizers were working with authorities to determine what happened.

The Associated Press said late Monday afternoon that at least two unexploded devices were found and being dismantled in Boston.

In a news conference, Boston Police Commissioner Ed Davis said a third explosion had taken place, at the JFK Library. Davis said he was not certain if the JFK Library incident was related, but that authorities are treating it as if it is. The Boston Police later said it was an unrelated fire.

Davis called the situation an ongoing event and advised people to make their way home or back to their hotels.

Officials advised people not to congregate in large groups in or around the area.

A reporter for WHDH says that multiple injuries have occurred at a building adjacent to the area. The Associated Press reports bloody spectators were being carried Monday to the medical tent that had been set up to care for fatigued runners. Police wove through competitors as they ran back toward the course.

Jackie Bruno, reporter for NBC-owned New England Cable News, said on Twitter that she saw people's legs blown off.

"Runners were coming in and saw unspeakable horror," she said.

Dr. Alisdair Conn, chief of emergency medicine at Massachusetts General Hospital, characterized the blasts as a military-style bombing.

"This is like a bomb explosion we hear about in the news in Baghdad or Israel," he told reporters.

"There are a lot of people down," said one man, whose bib No. 17528 identified him as Frank Deruyter of North Carolina. He was not injured, but marathon workers were carrying one woman, who did not appear to be a runner, to the medical area as blood gushed from her leg. A Boston police officer was wheeled from the course with a leg injury that was bleeding.

The American Red Cross and Googlehave both set up person-finder sites to provide information for individuals trying to track down friends or relatives who may have been near the finish line.

Will Ritter, the spokesman for a Massachusetts Senate candidate, told NBC News that he heard what sounded like two explosions and saw smoke rising near the Boston Public Library. He said that he saw three fire engines and police running to the site.

"We heard two really large explosions in rapid succession, about a second apart from each other," Ritter said. "Everybody kind of ducked and hit the ground."

President Obama, speaking early Monday evening from the White House, said, "We will find out who did this and hold them accountable."

A House Homeland Security Committee official told NBC News that Boston firefighters reported discovering an undetonated device at an intersection near the finish.

The Boston police bomb squad was en route. Boston police confirmed there was an incident but did not immediately give details. Police, fire and medical technicians were responding. New England Cable News reported that the emergency response came within seconds.

Federal authorities told NBC News that they had no immediate information. In New York, a spokesman told Reuters that police were deploying counterterrorism vehicles around landmarks, including prominent hotels.

The Associated Press says cell phone service in parts of Boston
have been shut down to prevent possible remote detonations of additional explosive devices.

Police in New York City and London are stepping up security following explosions at the Boston Marathon.

Chief NYPD spokesman Paul Browne said critical response teams are deployed around the city until more about the explosion is learned. Officials are stepping up security at hotels and other prominent locations.

British police also say they are reviewing security plans for Sunday's London Marathon. It's the next major international marathon. A London Metropolitan Police spokesman says police are working with marathon officials to review security plans.

Visitors are being cleared from around the area of the White House as a precaution, said NBC's Chuck Todd. Todd says the area in front of the White House along Pennsylvania Avenue in Washington has been cleared and lined with security tape.

The National Hockey League announced late Monday afternoon that Monday night's game between the Ottawa Senators and the Boston Bruins, set to be played at Boston's TD Garden, has been postponed.

Later Monday in a press conference, Boston police said the investigation is still very active. They have yet to confirm the number of explosives that were placed near the finish line of the race.

Police said this case is not going to be easy or regular, and that the area will remain closed for the night. They are investigating the properties along Boylston Street.